Microwave heating



Aug. 25, 1953 T. P. KINN 2,650,291

MICROWAVE HEATING Filed oct. 4, 194e Patented Aug. 25, 1953 UNITEDSTATES PATENT Q'FFICE MICROWAVE HEATING Application October 4, 1946,Serial No. '701,305

(Cl. 21S- 47) 9 Claims. ll

This invention relates to micro-wave heating, and has particularrelation to apparatus for heating strips of material with ultra-highfrequency energy.

In many industries there are dielectric heating problems involving thedrying, curing or setting of resins or glues in continuous processes. Inthe textile industry, for example, cloth strips are to be dried afterdyeing and often on a continuous basis, that is, in an operation inwhich the strip is continuously moving. Similar drying operations arenecessary in the paper industry, where a continuous sheet is moved at arelatively high speed.

At conventional frequencies of electric energy in the usual highfrequency heating range, apparatus with enough power for the drying ofpaper or textiles or other material is available, but frequentlydifficulty is encountered in getting that power into the work. Referringto the familiar relation for power in a dielectric material, it isevident that power into the work, once the frequency is set, varies onlyas the square of the voltage across the electrodes through which thepower is transferred to the material in conventional high frequencyheating apparatus. Thus,

2 Power frequency X voltage loss factor For thin materials, such aspaper and cloth, it is impossible to apply sufficient voltage across theelectrodes at conventional frequencies as ilashover between theelectrodes takes place long before any appreciable amount of power iscoupled into the dielectric work material.

lt is, accordingly, an object of my invention to provide new andimproved apparatus for use in heating a moving strip of material.

Another object of my invention is to provide novel apparatus for heatinga continuously moving strip of thin dielectric material.

A further object or my invention is to provide novel apparatus forheating a continuously moving strip of dielectric material in whichuniform heating of the material is accomplished with a relatively smallexpenditure of energy.

Still another object of my invention is to provide novel apparatus forheating a moving strip of material quite rapidly while avoiding anyfiashover through the material.

In accordance with my invention, l propose to heat a moving strip ofmaterial with appara.- tus which comprises a cavity resonator adapted tohave high frequency electromagnetic field oscillations establishedtherein, the resonator being positioned so that the strip is fedtherethrough with the plane of the strip substantially parallel to thedirection of the electric field within the resonator. However, the pathof nio-vement of a point on the strip through the resonator is arrangedto be angular with respect to the line of the electric eld vector,representing the direction of the field, so as to cause any given pointon the strip to progress an even number of half wave length along andwithin the interior of the resonator in a component directionsubstantially perpendicular to the electric field vector. By thisarrangement rapid and uniform heating of the strip of material isaccomplished without danger of ashover.

rlhe features of my invention which are believed to be novel are setforth with more particularity in the accompanying claims. The inventionitself together with additional objects and advantages thereof may bebetter understood from the following description of a specificembodiment when read in connection with the accompanying drawing, inwhich the single figure is a perspective view of apparatus embodying myinvention.

As shown in the drawing, a hollow rectangular envelope forms a cavityresonator 3 which is adapted to have high frequency electromagneticfield oscillations established therein. This energy may be introducedinto the resonator by any suitable means as, for example, through anopening -5 in a wall of the resonator through which electromagneticenergy is supplied from a hollow wave guide l. The wave guide, turn, mayreceive energy from any suitable high fre-- quency source and, ofcourse, may be provide-:l with suitable matching apparatus.

The electromagnetic field within the resonator may have an electric leldvector extending in one direction as represente-d by the arrow il. Apair of slots Il and I3 are provided in opposite walls of the resonator3 to receive a strip of dielectric material |55. The strip i5 is to becontinuously moved through the resonator1 by any suitable driving means(not shown) connected to suitable rollers il. The slots il and i3 are solocated that the plane of the strip iii is sul:-

stantially parallel to the arrow 9 repress: ing the direction of theelectric field. In ion,

the strip. In the embodiment shown, such maximum intensities are foundalong a center line of the resonator, in the plane of the strip. Thus,as the strip of material passes through the resonator, it is subjectedto the strong dielectric field which provides a rapid heating of thestrip.

There will, of course, be a lengthwise standing wave effect along thelength of the resonator 3 in the plane of the strip. Because of thisstanding wave effect, the material would be heated unevenly if passedstraight through the resonator with the direction of motion parallel tothe electric field vector. l-lowever, the slots ll and i3 have theircenter lines displaced by approximately n half wave lengths, where n isa whole number, preferably one-half wave length, in a directionsubstantially perpendicular to the electric field vector. Consequently,any given point on the strip i5, in passing through the res-- onator 3,also progresses approximately n half wave lengths along and within theinterior of the res-onator in a direction substantially perpendicularthe electric field vector. es a result, the standing waves exert auniform effect over the entire width of the strip i5 with each point ofstrip being subjected to the same integrated heating so that uniformheating is obtained. This uniform heating is obtained by having eachmoving point of the strip pass through the same range of field strengthsbetween minimum and maximum field intensities of the lengthwise standingwave. A distance or displacement of a quarter-wave length includes thisrange; but in the preferred embodiment described, each point passesthrough such range twice (one-half wave) for greater assurance ofuniform heating.

Although I have shown and described a preferred embodiment of myinvention, l am aware that many modifications thereof be withoutdeparting from the spirit of the invention. I do not intend, therefore,to limit ny invention to the specic embodiment disclosed.

claim as my invention:

l. Apparatus for use in heating a moving elongated insulating materialcomprising a hollow envelope providing a cavity resonator to havefrequency electromagnetic field oscillations having an electric fieldvector in one direction established therein, said rez nator havingopenings in different sides thereof, and means for guiding said stripthrough said openings and through said resonator with the materialsubstantially parallel to the vector of the electric field and with thepath of movement of said material through said resonator such as tocause any given point on the material to progress approximately nhalf-wave lengths along and within the interior of the resonator in adirection. substantially perpendicular to said electric field vector,where n is a whole number.

2. Apparatus for use in heating a moving strip insulating materialcomprising a hollow envelope providing' a cavity resonator adapted tohave high frequency electromagnetic field oscillations establishedtherein having an electric field vector in one direction, said resonatorhaving a pair of slots therein on opposite sides thereof, and means forfeeding said strip in one of said slots and out the other, said slotsbeing located to that the plane of said strip is substantially parallelwith said electric eld vector with the centers of the slots displacedsubstantially one half-wave length in a direction substantiallyperpendicular to said electric eld vector, any

given point on said strip progressing in a direction substantiallyperpendicular to said electric field vector as the strip is fed throughthe cavity resonator.

3. Apparatus for heating an elongated insulating material, comprising ahollow envelope providing a cavity resonator adapted to havehighfrequency electromagnetic field oscillations established therein anda standing wave with an electric held vector in one direction, saidresonator having opposite Sides, each of said sides having a holetherein, said holes being in registry in a plane including saiddirection, and means for guiding said material into one of said holesand out of the other, said holes being displaced in a directionperpendicular to said direction of said electric field, the displacementbeing at least a distance substantially equal to a quarter-wave lengthof said standing wave, any given point on said material progressing in adirection substantially perpendicular to said electric field vector asthe material passes through the cavity resonator.

4;. Apparatus for use in heating a moving strip of insulating material,comprising a hollow enelope providing a cavity resonator adapted to havehigh-frequency electromagnetic oscil- "ia-tions established therein withan electric I'coto in direction, said resonator having sides, each ofsaid sides having a slot g entirely within and from the edges side, andmeans for feeding said 'ip in one of said slots and out of the other, cslots lying substantially in a plane substantially parallel to saiddirection of the electric Q nd provided by said oscillations, and saidstrip being fed through the cavity resonator such that any given pointon the strip progresses in a direction substantially perpendicular tothe electric field vector and substantially parallel to the direction ofsaid slots.

5. An invention comprising that of claim 4, but further characterized bysaid slots being located in said opposite walls with their midpointsdisplaced in a direction lying in the associated side.

6. in invention including that of claim 5 but characterized by saidslots comprising portions `of a size corresponding to the strip passingtherethrough, said portions being displaced by substantially n half-wavelengths in a direction parallel to their lengths.

'2. Apparatus for use in heating a s rip of insulating material,comprising a hollow envelope providing a cavity7 resonator yadapted tohave high-frequency electromagnetic field oscillations establishedtherein providing a standing wave therein, said resonator having a pairof opposite sides, each of said sides having a slot, said slots beingsubstantially ccplanar, means for feeding said strip through saidresonator, through one of said slots and out of the other, said slotsbeing constructed and arranged in said opposite sides so as to bedisplaced from each other a distance substantially egual to at least aquarter of a wave length of the standing wave as a reference, said slotsbeing of substantially the same size as the width of said strip withsufcient clearance being provided between the slots and said strip suchthat the strip freely passes through the slots.

8. An ultra high frequency heater co crising walls made of anelectrically conducting material forming a chamber provided withdiagonally opposite openings in its ends, means for pass- 5 ing amaterial to be heated diagonally through Said chamber between saidopenings, and a high frequency supply source connected to said chamberfor producing an electric field mode in said chamber thereby to heat thematerial uniformly as it passes through said chamber.

9. An ultra high frequency heater comprising Walls made of anelectrically conducting material forming a chamber provided withdiagonally opposite openings in its ends, means for passing a materialto be heated diagonally through said chamber between said openings, anda high frequency supply source connected to said chamber for producing atransverse electric eld mode in said chamber thereby to heat thematerial uniformly as it passes through said chamber.

THEODORE P. KINN.

References Cited in the le 0f this patent UNITED STATES PATENTS NumberNumber Name Date 2,197,122 Bowen Apr. 16, 1940 2,226,871 Nicholas Dec.31, 194C- 2,364,526 Hansell Dec. 5, 1944 2,370,161 Hansen Feb. 27, 19452,398,606 Wang Apr. 16, 1946 2,400,777 Okress May 21, 1946 2,427,094Evans Sept. 9, 1947 2,433,067 Russell Dec. 23, 1947 2,500,752 Hanson etal Mar. 14, 1950 FOREIGN PATENTS Number Country Date 417,564 GreatBritain Apr. 18, 1934 518,691 Great Britain Mar. 5, 1940 OTHERREFERENCES- Skilling, Physical Behavior of Wave Guides, Electronics,March 1943, pages 76-80 inclusive. Electronic Heat, Steel, November 12,1945, page 92.

